The rate at which water permeates through the filter paper varies depending on the condition of the sludge and the filterability of the cake formed on the filter paper. The CST is obtained from two electrodes placed at a standard interval from the funnel. The time taken for the water front to pass between these two electrodes constitutes the CST.

The force generated by capillary suction is much greater than the hydrostatic head within the funnel, so the test is independent of the amount of sludge, as long as there is sufficient to generate the CST.

SSVI Equipment
The Stirred Specific Volume Index (SSVI) is used to determine the settling properties of an activated sludge. It is a simple test that is easy and quick to perform, and yet provides considerable information about the sludge settlement characteristics.
The SSVI can be used both in routine plant operation and it can be used for the design of a final sedimentation tank.

Triton - Model Type 304B - Capillary Suction Time Unit (CST) Brochure